The present invention relates to a drive circuit for a piezo-electric element, and in particular to a drive circuit for a piezo-electric element which can provide stable drive voltage to said piezo-electric element and also a fast response.
The use of a piezo-electric element as a means for converting electric power into mechanical power has been known from the past. For instance, such a piezo-electric element may be conveniently used as a drive means in a relay. This is convenient, since a piezo-electric element is simpler in structure and requires less power than a more conventional electromagnetic solenoid.
However, since a piezo-electric element requires a relatively high drive voltage, a special power source therefor which can supply a sufficiently high voltage is required. When such a high voltage is not directly available, a drive circuit which can boost up the drive voltage to the required high level is required.
FIG. 1 of the accompanying drawings is an electric circuit diagram of a conventional drive circuit for a piezo-electric element. A voltage supplied from a DC power source E is supplied to a blocking oscillator H which produces an output consisting of alternating current of a higher voltage. The output of the blocking oscillator circuit H is rectified by a diode D3 and then supplied to a piezo-electric element P.
This drive circuit can provide the voltage required for driving the piezo-electric element P, but has the following shortcomings:
Although the drive circuit is provided with a zener diode ZD1 for stabilizing the input voltage to the blocking oscillator circuit H, fluctuation in the voltage of the power source E still affects the blocking oscillator circuit H and causes changes in the output voltage. The piezo-electric element P normally has a permissible range of drive voltage which may be applied thereto. If the output voltage of the blocking oscillator circuit H is too low then the piezo-electric element P may not deform enough for its function, while if this output voltage is too high it may damage the piezo-electric element P and thereby reduce its reliability.
Also, when the piezo-electric element P is to be deactivated, the electric charges stored in the element must be discharged to ensure its restoration to the deactivated state. A resistor R2 is provided for this electric discharge function, and the resistance of this resistor R2 determines the time constant of the discharge circuit for the piezo-electric element P. Therefore, the smaller the value of the resistor R2, the faster the piezo-electric element P can perform its function. On the other hand, since the resistor R2 is connected in parallel with the piezo-electric element, it has the effect of being a dead load for the blocking oscillator circuit H when the piezo-electric element P is being activated, and thus increases the overall power consumption.